1. Field of the Invention
The present invention relates to a sheet processing apparatus including a mechanism of folding a sheet-like medium such as, for example, a sheet.
2. Description of the Related Art
Hitherto, as a sheet folding mechanism to fold a sheet, for example, FIG. 1 of U.S. Pat. No. 4,917,366 shows a folding unit provided between a copying machine and a finisher unit, and FIG. 2 shows a folding device and sheet feed paths.
Further, as shown in FIG. 17, a Z-folding mechanism 100 is known in which roller pairs are arranged and a method of folding a sheet simultaneously with transport thereof is used. The Z-folding mechanism 100 includes a pair of feed rollers 101 and 102 to transport a sheet through a nip thereof at a constant speed in an arrow T0 direction (left direction in the drawing), a plate-like transport guide 103 to guide the sheet nipped and transported by the pair of feed rollers 101 and 102 in the horizontal direction, a stopper 104 with which the leading edge of the sheet collides at the end position of the transport guide 103 and which regulates the movement thereof, a feed roller 105 disposed rotatably in contact with the left side, in the drawing, of the one feed roller 101 in the vicinity of the start end of the transport guide 103, a feed roller 106 disposed rotatably in contact with the upper side, in the drawing, of the feed roller 105, a plate-like transport guide 107 to guide, in the vertical direction, the sheet fed upward in the drawing through a nip between the two feed rollers 101 and 105, and a stopper 108 with which the sheet collides at the end position of the transport guide 107 and which regulates the movement thereof. The four feed rollers 101, 102, 105 and 106 are always respectively rotated together at the same peripheral speed in the arrow directions in the drawing.
The Z-folding mechanism 100 operates as follows. First, the sheet is fed from the arrow T1 direction in the drawing to the nip between the feed rollers 101 and 102, and is transported along the transport guide 103 in the left direction in the drawing. After the leading edge of the sheet in the transport direction collides with the stopper 104, the sheet is further fed in the left direction in the drawing by the feed rollers 101 and 102, and a halfway portion of the sheet, which has nowhere to go, is warped and deformed by buckling.
Further, a convex side portion of the warped and deformed sheet escapes toward the nip between the feed rollers 101 and 105, is nipped and restrained by the feed rollers 101 and 105, and is transported upward in the drawing along the transport guide 107. At this time, the warped and deformed portion of the sheet passes through the nip between the feed rollers 101 and 105, so that the sheet is folded.
The leading edge, in the transport direction, of the sheet transported upward in the drawing along the transport guide 107 while the folded position is made the head, that is, the folded position collides with the stopper 108, and then, the sheet is further transported by the feed rollers 101 and 105, and the halfway portion of the sheet including, in this case, the leading edge of the sheet is warped and deformed by buckling.
The portion of the sheet, which is buckled and has nowhere to go, escapes toward the nip between the feed rollers 105 and 106 as shown in the drawing, is nipped and restrained by the feed rollers 105 and 106, and is fed toward the arrow T2 direction in the drawing. By the foregoing series of operations, the sheet is folded into Z-shape.
In the folding device as stated above, for miniaturization and simplification of the structure, the diameters of the rollers used for the feed rollers and the first and the second folding rollers are equal to one another, the feed directions of the feed rollers and the first folding rollers are made orthogonal to each other, and the feed direction of the first folding rollers and the feed direction of the second folding rollers are made orthogonal to each other, so that the miniaturization of the apparatus and the improvement of feed performance of the roller pairs are realized.
However, in the foregoing conventional Z-folding mechanism 100, since the leading edge of the sheet in the transport direction is collided with the stopper 104, 108 and the sheet is warped and deformed by buckling, there is a case where the sheet can not be well buckled at a desired folded position according to the kind of the medium, for example, a paper having a relatively large size or a thin paper having a relatively low firmness, and the deformation occurs at a different portion. Accordingly, it is difficult to accurately obtain a fold line position.